A spoked bicycle wheel conventionally comprises a rim, a hub and a set of spoke connections between the rim and the hub.
More specifically, for the connection between rim and hub, a spoke having a widened head at a first end, and an outer threading at a second end opposite the first end is typically provided.
The widened head of the spoke is received in a suitably shaped seat, usually made at the hub or at a flange thereof. In order to allow the extension of the spoke in the radial or substantially radial direction, a bend is typically provided close to the first end of the spoke. The bend of the spoke at the first end can, however, be left out.
The threading at the second end of the spoke is coupled in a threaded hole usually made at the rim or, preferably, in a removable connection element of the nipple type or nut type, which abuts at seating holes in the rim, possibly through the interposition of a plate.
Spokes with different attachment means to the rim and to the hub are also known, for example where the head of the spoke is intended to couple with the rim, and the threading of the spoke is intended to couple with the hub. The present invention is not limited to any particular type of spoke connection.
In the present invention and in the attached claims, the expression “spoke connection” is meant to indicate the set of a spoke and possible removable connection elements, such as the aforementioned nipple and interposition plate.
The hub is an element with rotational symmetry with respect to the axis of the bicycle wheel, and the spoke connections are always so distributed along the circumference of the wheel that the center of mass of them as a whole is at the axis of the wheel. More specifically, the spoke connections can be distributed equally spaced apart, or can be distributed equally spaced apart along the rim in groups of two or more spoke connections.
Due to the different angle of the spoke connections with respect to the median plane of the wheel (camber) between one side and the other of the hub and/or due to a tangential attachment of the spokes to a side of the hub and/or for other reasons in particularly complex spoke patterns, there can be spoke connections of varying masses, but still forming regular, with respect to the rotation axis, sub-sets of identical spoke connections. Also in such cases, the center of mass of the set of spoke connections is therefore at the rotation axis of the wheel, in other words the set of spoke connections is balanced with respect to the rotation axis.
It is known to make rims with a low wall thickness in all of the zones between the spokes, or at least in all of the zones between groups of spokes that are close together, in order to contain the weight of the rim while still offering adequate structural strength along the whole rim.
The known geometries of the rims are such that also the rim is an element with rotational symmetry with respect to the axis of the wheel.
At first view, the wheel is therefore dynamically balanced.
In practice, however, there is always a cause for an imbalance or a mass discontinuity, in particular a localized mass increase, caused by the valve that retains the air inside the pneumatic tire, be it of the type with an inner tube, of the tubeless type or of the type with a tubular tire. The valve, fixed in a proper hole formed in the rim, is typically made of brass and has standard dimensions and therefore predetermined weight. The slight lightening of the rim as a result of the housing hole provided for the valve is insufficient to compensate for the localized mass increase of the valve itself. On the contrary, in rims obtained by carbon fiber molding, the localized mass increase at the valve is sometimes accompanied by a further increase in mass due to the presence of a greater thickness of the rim in such a zone in order to compensate for the weakening of the rim due to the hole for the valve.
The cause for imbalance provided by the valve is accompanied, in the case of metal rims, by a second cause. Metal rims, in particular those made of steel or aluminum alloy, are made through a rod extruded according to the desired section of the rim, said rod then being shaped into a circumference by calendaring. The jointing of the ends of the rod is carried out in various ways, all of which however provide for the addition of material and therefore determine a localized mass increase. For example, known methods for carrying out jointing provide for: the insertion with interference of one or more pins in respective holes butt formed in the wall of the ends of the extruded rod; the insertion of a sleeve in an inner chamber of the extruded rod for a certain extent from both ends, possibly with the addition of an adhesive; and the butt welding of the ends, carried out with or without addition of welding material, but in any case using solid metal inserts in the inner chamber of the extruded rod in order to allow for the gripping of the ends with suitable pincers during welding without the risk of deforming the rim.
One or both of the causes for localized mass increase bring about that the center of mass of the masses does not belong to the rotation axis of the wheel. From a dynamic point of view, the imbalance caused by such localized mass increases brings about an unfavorable instability of the wheel. Moreover, given that the imbalance increases as the speed of the wheel increases, it becomes more dangerous precisely when a very stable wheel is needed, i.e., when the travel speed is fast, like when going downhill.
In the case of metal rims, to reduce the problem of dynamic imbalance, it is known to make the hole for the valve in a position diametrically opposite the joint of the extruded rod. The balancing of the masses in movement thus obtained is, however, insufficient to provide good stability of the wheel since typically the additional mass in the zone of the joint is different from, normally greater than, the mass of the valve body. Moreover, in the case of wheels with odd spoke patterns, it is sometimes not possible to make the hole for the valve in a position diametrically opposite the joint of the extruded rod, but only in an approximately opposite position since the diametrically opposite position is occupied by a spoke or by a group of spokes. The resultant of the forces cannot, therefore, be cancelled out due to the aforementioned mass difference and/or due to the angle that the centrifugal forces due to the two localized mass increases form between them.
It has also been attempted to solve the problem of balancing in such rims by sticking a plate at the valve hole. By providing for an additional element, such a provision is however unsatisfactory both from the manufacture point of view and from the point of view of the end product, since, besides increasing the weight of the rim, the plate risks detaching with the use of the bicycle, is unaesthetic and not aerodynamic.
The technical problem at the basis of the present invention is to effectively reduce the dynamic imbalance of a bicycle wheel.